Seminal work in this field is memorialized in 1964 U.S. Pat. No. 3,161,442 of Frank A. Reed. He devised a rotary feed structure in the form of a segmented rotating bowl, to carry particulate material from a hopper to a delivery tube--with compressed-air expulsion of the material from U-shaped passageways or "pockets" of the feed bowl into the delivery tube.
Strengths of Reed's system are that only one relatively small area about the expulsion station must be sealed to constrain the compressed air and the material, and gravity tends to retain the material in the bowl (thus minimizing deterioration of the seal) until the material reaches the expulsion station. The sealing is accomplished by a thick elastomeric body--called a "pad"--of segmental shape. Apertures are formed in the pad for attachment of a pressurized-air fitting and the delivery tube, so that as each particular pocket in succession rotates into the expulsion station the particulate is blown out of that pocket through the pad and into the delivery tube.
The pad is cast-bonded to a metal top plate, which receives sealing force from cams above the plate edges and helps distribute this force over the pad area. Some sealing action around the edge of the pad is provided by a peripheral (except at the outer face of the machine) ridge or lip that engages adjacent radial, vertical walls of the mechanism. The sealing lip essentially helps to maintain separation of the inside of the machine from the outside--in the segmental area occupied by the pad--to minimize escape of dust and particulate at the edges of that segmental area.
During use the bottom of the pad is ground off by the abrasive particulates in the gunite, and the pad correspondingly descends. To accommodate this motion the cams extend and the lip slides downward along the radial vertical walls.
Analogous European developments aimed for different benefits, settling on a straight-through rotary structure that tends to be clog resistant (though not entirely immune) because it is gravity-assist-emptied as well as gravity-filled, and because the particulate material need not negotiate hairpin curves within the feed chambers.
The European configurations, however, are inherently bulkier vertically and employ two very large seals, at top and bottom respectively. The highly abrasive material to be dispensed is gravity-loaded against the lower seal, which is therefore subject to extreme wear.
Reed's patent too is hereby fully incorporated by reference into this document, as is 1991 U.S. Pat. No. 5,150,991--coowned with the present document by Reed Manufacturing, Inc.--which introduced very important refinements directed to control of the compressed-air-and-material sealing system.
Smooth flow--Issues of susceptibility to clogging were generally put to rest for many years by careful early design of passageways in the Reed cast-iron bowl, and also by controlling the direction of loading of relatively large rock or gravel pieces. In this regard, in a Reed machine a so-called "rock cone" is sometimes disposed at the axis of rotation.
The rock cone blocks the relatively large particulate from loading into the air-input end of each chamber: in the Reed system, the input ends are arrayed closely about the axis, and the output ends are arrayed nearer the periphery of the rotary bowl. Due to the radiating geometry of the system, the input ends are much more tightly constricted; and the cone is thought to help deter mechanical jamming of heavier aggregate in these input ends.
The rock cone is longitudinally slotted, to permit finer particulate material from the hopper to pass through and load directly into the air-input end of each chamber. This finer material poses no threat of mechanically jamming the input.
Recently, however, these flow issues have reappeared because of modern developments in gunite materials. This is true, in particular, of certain materials which have entered into favor for underground work, e. g., in mines and subway tunnels, where waterproofing is critical and development of small defects can create monumental damage.
For such applications, a material that tends to fill tiny gaps and enhance waterproofing is highly prized. Just such a material, for instance, is so-called "silica fume"--finely divided and somewhat resembling talcum. Previously regarded as a waste product, silica fume is currently popular, for its characteristics just outlined, as a constituent of particulate material for underground gunite use.
Unfortunately this same material also tends to create or aggravate limitations of the original design concepts of rotary feed structures. Silica fume and like materials when slightly moist tend to adhere strongly to cast-iron surfaces, partially set, and thus build up in a delivery system and choke off the passageways in the feed structure. (We here refer to build-up on internal surfaces of the passageways, not on the seal-facing surface discussed earlier in connection with abrasion.) This difficulty has been notable especially, but not exclusively, in the U-pockets of the Reed bowl.
Under field conditions, furthermore, removing the accumulated material is difficult. It can be chipped away, as for instance with a hammer and chisel--but sometimes a worker cannot plainly tell the difference between chipping gunite and chipping cast iron. In any event, as a matter of practical field operations, the efforts required to keep a rotary feed structure clear enough for effective use are often excessive.
Air quality--An analogous historical development can be traced in the area of dust control. Both the Reed and the European machines initially had a tendency to emit dust, because each feed chamber emerging from the expulsion station is still partly pressurized--typically at more than 200 kPa (30 psi).
If such a chamber rotates directly back under the particulate-material hopper to receive another load of material, this residual pressure is at least partially relieved into the hopper. It tends to blow, backward and upward out of the hopper, whatever relatively fine material is present.
This dust emission was a lesser or greater problem depending upon various factors. These included especially the specific type and dryness of material in use, duration of the application session, degree to which the work space and associated areas were confined, and sensitivity of the particular environment (for instance highway, industrial, commercial, or residential) to the esthetics of dust pollution. Where the overall combinations of these factors were relatively less sensitive, dust emission was ignored.
In the relatively more sensitive combinations of these factors, some users developed their own arrangements for mitigation. Such provisions generally took the form of providing some sort of plenum or house, in a region following the expulsion station, to recapture the residual pressurization, and divert it away from the input hopper.
Dust as a pollutant, however, like clogging became a more significant problem with the advent of silica fume as a favored constituent. This material, being finely divided in the extreme, is far more susceptible to becoming airborne.
As before this occurs under the influence of air-pressure residuals after feed-structure chambers have been, in principle, pneumatically emptied. With silica fume, however, merely isolating and venting the residual pressurized air cannot adequately control the situation, because that pressurized air is now directly laden with ultrafine silica-fume silt left in the chamber at the expulsion station.
The finely divided character of this same material also renders it more susceptible to escape through relatively small passageways in the air-pressure sealing system, aggravating its pernicious tendency to float away on relatively small pressure residuals. The overall dust problem posed by silica fume is major when the material is moist, and worse when it is dry.
Therefore a necessity arises to somehow capture and deal with not only the pressurized air but also its entrained silica fume.
Prior mitigation work--Various efforts have been made in this field to overcome or at least reduce the problems arising from use of silica fume and the like. The above-mentioned parent patent document of the present document teaches apparatus and methods for essentially eliminating the flow problems.
As to dust generation, it was natural to attack the special problems of silica fume with familiar tools such as filters and collection chambers. Silica fume, however--by virtue of both its surprisingly high volume and its intimate association with the pressurization itself--turns out to be astonishingly difficult to manage.
Reasonably ample collection bags rapidly overfill, and persistently puff this powder through every seam and crevice. Ordinary filters, if fine enough to be at all useful, quickly clog. The material and the air which carries it are not readily separated from each other.
The resulting fine-airborne-dust problem is particularly insidious in the very environments where silica fume is most highly valued. Miners and other workers in connected cavities--even sometimes when quite remote from a gunite-shooting site--are highly vulnerable to air contamination by such fine particles.
As mentioned earlier, silica fume is particularly troublesome as to dust generation when the material is dry. In some environments this problem can be reduced slightly by using a predampening technique, which essentially consists of spraying the gunite material with quite small amounts of water in a rotating cement mixer before the material enters the hopper.
Such mitigations are not regarded as practical in the context of a mine. Gunite is typically brought into a mine in closed sacks.
Even handling of the sealed sacks of silica fume generates clouds of dust. A sack is attached to the top of the gunite-gun hopper, and then slit open by the machinery. This arrangement is considered necessary to minimize dust generation in transfer of the material from bag to hopper.
Underground operational conditions are cramped and remote, tending to militate against bringing in still an additional piece of equipment (such as a cement mixer). It is for all these reasons that in mining the technique of premoistening is considered infeasible and the dust-control problem has persisted at its worst.
Other efforts at dust control have attempted to take down the air-entrained gunite particles with a water scrubber. We are aware of some gunite-gun products, now on the market for some while, using a so-called "water ring"--a length of pipe formed into a toroid, with water holes drilled through the pipe wall facing toward the center of the toroid.
Such systems do greatly reduce the amount of dust released to the atmosphere. The amount of water used by a commercial prior-art scrubber for a gunite gun, however, is considerable.
The result is to release--usually onto the ground--correspondingly large amounts of very dilute gunite slurry. Most typically the amount of this contaminated water is excessive: it is large enough to be at least troublesome, if not somewhat impractical, to capture and haul away.
With extended operation the result is a notable mess. In unusual cases of very protracted operation, the initial air-pollution problem may even be replaced by a significant water-pollution or waste-disposal problem.
We have also noted that the commercially observed unit mentioned above has a relatively small-diameter exhaust line, terminating in an even smaller constriction. The use of a pneumatic impedance under certain circumstances can be very useful, but in the unit discussed here the small diameter and constriction appear to contribute only back-pressure in the exhaust plenum, and therefore to be counterproductive.
Although the water scrubber has now been accepted by the gunite industry for an extended period of time, it appears to us that this acceptance has been reluctant, and essentially as a last resort. By any other measure, prior-art scrubbers create--because of the amount of water used and discharged--an unprofessional and almost slovenly impression, which would be rejected commercially were it not for the crucial need for dust control.
Thus the need for a well-contained, refined resolution of the dust-emission problem in sensitive environments was long felt, but unmet until the advent of the innovations described in the aforementioned parent patent document. That document describes techniques for controlling both the amount of dust available for entrainment in the exhaust air of a gunite conveying machine, and the pressure with which that dust is expelled from the machine.
In the parent patent document it is shown in detail how the seal which retains the gunite in the feed bowl can be extended to cover the material-exhaust end of each feed chamber in the bowl. In such an arrangement, the material which does not quite manage to escape from the chamber in each delivery cycle is momentarily trapped at the exhaust end of the chamber--and during that moment the pressurization in the chamber can be relieved at the opposite, or air-intake, end of the chamber.
When these strategies are adopted, the trapped dust is thus behind almost all of the pressurized air that escapes from the open air-intake end of the chamber, so that very little dust is in a position to be expelled with and ahead of that escaping pressurized air. Dust emission is thereby dramatically reduced.
In the parent document it is also explained that there are two competing theories which could optimally implement strategies. In one approach the pad--where it extends into an exhaust plenum which follows the expulsion station--is placed in a relatively tight sealing position against the wear ring that is at the top of the feed bowl.
The object is to maximize the beneficial effects described just above. The extended pad, in this approach as described in the parent document, is formed with an additional through-aperture above the air-intake end of the feed chamber that has just passed away from the expulsion station.
In a competing approach, the material-exhaust end of the feed chamber--in that same exhaust plenum region--is guarded not by the tightly pressing pad but by a metal baffle or plate. This panel is actually stood off from the wear ring by some one or two centimeters (i. e., a significant fraction of an inch).
The rationale of this approach is that very tight sealing tends to develop high-pressure particulate leaks that can damage a nearby felt seal or other equipment, and may actually exacerbate leakage. In this approach the baffle is a separately formed metal piece, integral with or fixed to the plenum housing or other metal structure in the plenum region.
In both of these approaches the plenum was configured as a rectangular box, enclosing the pad extension or the stood-off baffle. At the time of preparation of the parent patent document--although the appropriate implementation was becoming clear--no entirely stable design and configuration of the plenum, or of the baffle or the pad extension, had then been achieved.
Thus in the parent document it is said, "we believe that far more effective dust control can be achieved by . . . venting the input end of the chamber through a port in the pressure seal; however, this may require a more extensive reconfiguration of the system, to lengthen the seal in the circumferential direction so that venting can be performed at a point that is safely isolated from the air-pressure source."
The previously mentioned U.S. Pat. No. 5,150,991 teaches substitution of a fluidic control system, in place of the theretofore-employed mechanical screws or levers, for operating the force-applying cams that press the pad against the wear plate of the rotary feed structure. The benefits of that general teaching have been well confirmed in the intervening years.
U.S. Pat. No. 5,150,991 also teaches that the fluidic system advantageously includes a first, pneumatic stage and a second, hydraulic stage--the former particularly contributing easy and delicate control of force levels, and the second particularly contributing positive and rugged application of the resulting forces. In addition the '991 patent teaches use of a check valve in the hydraulic stage, to prevent kickback of high force transients from the gunite-handling mechanical subsystem into the control system.
Although in general this dual system with the check valve has served well, it has in some installations proven slightly temperamental. In addition it is of course somewhat costly.
As can now be seen, the related art remains subject to significant problems, and the efforts outlined above--though praiseworthy--have left room for considerable refinement.